transmitted through crystallized Bodies* 
211 
plane of reflection, x\\e first set is still visible; but when the 
All the preceding observations were made with a plate of 
topaz of an inch thick. When the plate has a greater 
thickness the rings are much smaller, and when it has a less 
thickness the rings are extremely large, so that in very thin 
plates, only a small portion of a ring can be perceived at once. 
We have already seen that with a plate 
thick, the fourth red ring subtends an angle of i8° 30'. With 
another plate of an inch thick, the angle subtended by 
the same ring is 8° 24'. But since 
it follows that the conjugate diameters ofi the rings are inversely 
as the thickness of the plates. 
According to the Abbe Hauy, the angle formed by two of 
the primitive faces of the topaz is 124° 22'; and therefore the 
axes or longest diagonals of the primitive rectangular prism 
will form angles of 60° 31' 15"^ with a line perpendicular to 
the laminae, a result which agrees so nearly with 60® 38', the 
inclination of the axes of the coloured rings, that we can have 
no hesitation in concluding that the axes of the coloured rings 
are coincident zvitb the longest diagonals of the primitive rectan- 
gular prism. 
The rings which have now been described as produced by 
topaz, I have discovered in rock crystal, mica, the agate, the 
oriental ruby, the emerald, native hydrate of magnesia, amber, ice, 
sulphate of potash, tartrate of potash and soda, nitrate of potash, 
acetate of lead, acetate of lead melted and cooled, prussiate of pot- 
• According to my own measurements the angle is 123® 58^ which gives 60® 28' 
26" for the inclination of the diagonals. 
agate is turned round 90® the second set is perceived. 
= 18° 30' : 8* 24; 
E e 2 
